1. Field of the Invention
The present invention relates to an apparatus for displaying and controlling a picture signal, which has a PLE (Peak Luminance Enhancement) control means, for example, for finding an average brightness level (APL=Average Picture Level) of the picture signal to be displayed so as to control display brightness in an image display device by this average brightness level, and to a method of displaying and controlling the picture signal.
2. Description of the Related Art
For example, in PDP (plasma display panel) etc., the above-mentioned PLE control is carried out when displaying an image. This PLE control is arranged such that an average picture level (APL) of the picture signal corresponding to a field or the whole frame screen is detected, and a display brightness level which is a brightness level for actually displaying the image is set based on this average picture level.
In this case, as for the above-mentioned PLE control, in the case where the average picture level is low (or when the whole picture is dark) with respect to a signal having even the same brightness level, the display brightness level is set to be high so that high brightness display may be provided. On the other hand, in the case where the average picture level is high (when the whole picture is bright), the display brightness level is lowered so as to inhibit power consumption. By carrying out the PLE control in this way, it is possible to realize low power consumption and also possible to display an image of good contrast.
As described above, a display apparatus provided with a PLE control means which finds the average picture level APL of the picture signal to be displayed and controls the display brightness by this APL is disclosed in patent documents 1 and 2 as listed below, for example.    [Patent document 1] Japanese Patent Publication (KOKAI) No. H9-281927    [Patent document 2] Japanese Patent Publication (KOKAI) No. 2001-175220
When adjusting brightness of a display screen by using the above-mentioned PLE control means, an image memory for a plurality of screens (at least two frames) is generally needed in order to adjust the brightness of a display picture without delay by way of the PLE control. FIG. 1 shows a basic structure by a block diagram, reference sign A indicates a display control apparatus into which the picture signal is inputted, and reference sign B denotes an image display means for receiving the picture signal whose brightness is controlled by the above-mentioned display control apparatus A and displaying this picture signal.
The above-mentioned display control apparatus A is provided with first and second image memories 1a and 1b which can each write the picture signal for one frame. The picture signal to be inputted is first transmitted to the first image memory 1a (hereafter referred to as VRAMa), into which the picture signal for one frame is written. At the same time, the above-mentioned picture signal is transmitted to an APL unit (average brightness calculation means) 2, in which an average picture level (hereafter referred to as APLa) is calculated from the picture signal for the above-mentioned one frame. Then, information data on the average picture level APLa calculated in the APL unit 2 are sent to a brightness control unit (brightness control means) 3 as a control signal.
Subsequently, the picture signal of next frame is sent to the second image memory 1b (hereafter referred to as VRAMb) into which the picture signal for the above-mentioned next one-frame is written. At the same time, the picture signal for the above-mentioned next frame is sent to the APL unit 2, and an average picture level (hereafter referred to as APLb) is calculated from this picture signal for one frame.
During this time, the picture signal read from VRAMa is subjected to brightness control based on the average picture level APLa which corresponds to one previous frame in the brightness control unit 3, and is set as a display brightness level corresponding to the average picture level APLa. The picture signal for the previous one frame set as this display brightness level operates to be sent to the image display means B to display an image.
Then, the picture signal after being read from VRAMb is subjected to brightness control based on the average picture level APLb which corresponds to next one-frame in the brightness control unit 3, and is set as a display brightness level corresponding to the average picture level APLb. Similarly, the picture signal for the next one-frame set as this display brightness level is transmitted to the image display means B to display an image. That is, the above-mentioned operation is repeated one by one.
Incidentally, as shown in FIG. 1, the structure provided with the image memories for a plurality of frames leads to an increase in memory capacity. The increase in memory capacity brings about increasing costs of IC's which construct the memories, and results in increasing costs of a product inevitably. Therefore, for example, in a small display apparatus used for a portable telephone etc., it is often the case that a structure provided with an image memory for one frame as shown in FIG. 2 is employed, since it is difficult for the apparatus to have an image memory for a plurality of frames in view of its volume and cost.
In other words, reference numeral 1 shown in FIG. 2 is an image memory into which the picture signal for one frame can be written, and which corresponds to either one of the image memories indicated by reference signs 1a and 1b shown in FIG. 1 as already explained. Further, in a structure as shown in FIG. 2, parts performing the same functions as the already described parts in FIG. 1 are given identical reference signs. Accordingly, the description of these will not be repeated.
Since the image memory 1 used in the structure as shown in FIG. 2 has the capacity which allows the picture signal for one frame to be written in as described above, it operates so that a new picture signal is stored one by one into the image memory 1, while rewriting it (overwritten), as time goes on. FIG. 3 explains the operation, a horizontal axis shows elapsed time, and a vertical axis shows a progress state of write-in operation of writing the picture signal into the image memory (VRAM) 1. In addition, it can also be said that the vertical axis indicates an advance rate of a display scan.
As described above, as time goes on, with respect to the picture signal for the preceding (one frame before) old frame, the image memory 1 is overwritten by the picture signal for the following new frame one by one. Therefore, when trying to realize the above-mentioned PLE control, a delay of one frame period (at maximum) arises in the display brightness according to a display frame and APL. As a result, a display portion corresponding to the new frame takes place, which is displayed and controlled based on the average brightness of the old frame as shown by a dotted hatch part C in FIG. 3.
Now, for example, in the case where the picture signal of the old frame is of a dark image and the picture signal of the new frame is of a bright image, since the above-mentioned PLE control is carried out such that the brightness of the bright image of the new frame is controlled by APL based on the dark image of the old frame, it operates so that a level of each peak brightness of the bright image of the new frame may be further raised. For this reason, there arises a problem that a bright image is displayed still more brightly at an instant in time. At the same time, there arises another problem that an excessive load is momentarily applied to a power supply circuit, because large drive current may flow to each of the display pixels to be lit which have a high proportion of the image display means.